AST value in athletes: muscle or liver?

The assay counts how much AST is circulating in your blood at that moment, in units per litre (u/l). AST, in full aspartate aminotransferase and on older reports SGOT, is an enzyme that helps convert amino acids, the building blocks of protein, inside cells. As long as a cell is intact it stays neatly inside; if the cell is damaged it leaks into the blood. For an athlete the question is therefore simple: how many cells broke recently, and which tissue did they come from?

In trained people, skeletal muscle is often the largest source. Eccentric loading does this most strongly: the braking half of a movement, such as the descent of a squat, the landing in a jump or running downhill. Your soreness comes from there too. The enzyme also comes from heart muscle, the kidneys, brain tissue and red blood cells, and the moment liver cells are damaged it leaks out there just as readily. The assay cannot separate those sources; you make that distinction with the three values beside it.

Creatine kinase, CK for short, comes almost exclusively from muscle and is therefore your clearest signal. ALT is the liver-specific counterpart: if your AST is higher than your ALT, muscle is the likelier source. Gamma-GT notably does not move with muscle damage, and together with alkaline phosphatase it is your steadiest anchor. That fourth value is the most underrated instrument you have: if AST and ALT are both raised while gamma-GT stays normal, a liver cause is less likely than the alarm suggests.

The upper limit is 35 u/l for men and 31 u/l for women; men carry more muscle mass on average, and muscle supplies part of the circulating enzyme. Reference values differ per laboratory, so the range on your own report is the one that counts.

If you train hard, the question is rarely whether your AST is raised, but raised relative to what. Your baseline sits higher than that of someone who does not train, because you carry more muscle and it is damaged more often.

The sharpest research on this had untrained men do one heavy weightlifting session. Their transaminases, AST and ALT together, climbed to values resembling liver disease and were still elevated seven days later, while gamma-GT and bilirubin did not move. Seven days is far longer than the soreness lasts, and that is exactly why people are caught out.

A raised AST has more causes than training. Heavy alcohol use, fatty liver disease and some medicines belong there too, and certain medicines only lift the result after weeks. Look at the magnitude as well: a mildly raised value fits muscle or alcohol, while a value more than ten times the upper limit almost always points to acute viral hepatitis, and then you call your GP. In liver conditions such as hepatitis the ratio also tips and ALT rises above AST.

The reading grid is the heart of this page, and it is about four values at once. If AST is raised with a normal or slightly raised ALT, a raised CK and a normal gamma-GT, skeletal muscle after training is the most likely source. If AST is raised but ALT sits higher still, with a normal CK, it is the liver cells. If AST is raised, ALT sits lower so that the ratio exceeds 2, CK is normal and gamma-GT is raised, that is the alcohol pattern. And if AST is markedly raised with a normal ALT and a very high CK, there is extensive muscle breakdown, of which more below. So if AST and ALT are both raised after a heavy week, the first question is what CK and gamma-GT are doing.

What this value does not measure is your heart. AST used to be used for that and troponin took over that role long ago.

One warning, and it is not theoretical. With extreme exertion, heat or dehydration, or with an unusually large training stimulus, muscle tissue can break down massively. CK then climbs into the thousands of u/l, the muscles swell and hurt severely, the urine turns dark like cola and the kidneys are at risk. This is emergency care: do not order a blood test and do not wait for a result, but go straight to an out-of-hours GP service or emergency department.

When you draw matters more here than any other preparation. Draw during a deload week, because that is when your muscles are least busy repairing and a raised AST says something about your liver; draw mid-way through a heavy block and you mostly measure your training load. Leave at least 48 to 72 hours after the last heavy or unaccustomed session, and after a first exposure to a new movement, a competition or an eccentric programme preferably longer, because the effect can persist for a week.

Request CK alongside it. Without CK a raised AST is a puzzle, with CK included it is usually clear at a glance. Ask when AST should be measured again as well, because a repeat often says more than the first result.

Measure under comparable conditions. Your own series over time says more than a single value, so draw at the same point in your block each time. If AST stays raised even though you took the rest and CK is normal, discuss that result with your GP.